Abstract:

Systems, methods, and other embodiments associated with RE-TOSSI are
described. One system embodiment includes an MRI apparatus configured to
produce a RE-TOSSI pulse sequence and to acquire T2-weighted images in
response to the RE-TOSSI pulse sequence. An example RE-TOSSI pulse
sequence includes a TOSSI portion and a non-inverting, non-TOSSI portion.

Claims:

1. An apparatus, comprising:a first logic to produce a T1-insensitive
steady state imaging (TOSSI) pulse sequence having at least three
inversion pulses, where the spacing between the at least three inversion
pulses is not equal;a second logic to produce a non-inverting, non-TOSSI
pulse sequence, where the non-inverting, non-TOSSI pulse sequence does
not include inversion pulses associated with a TOSSI pulse sequence; anda
combination logic to provide a combined acquisition technique pulse
sequence comprising a first portion provided by the first logic and a
second portion provided by the second logic, where the first portion
precedes the second portion.

2. The apparatus of claim 1, where the at least three inversion pulses are
to be distributed in a low flip angle bSSFP acquisition.

3. The apparatus of claim 2, where the at least three inversion pulses are
configured to balance signal gained and lost via T1 relaxation in states
parallel and anti-parallel to a main magnetic field produced by an
imaging system controlled by the apparatus.

4. The apparatus of claim 1, where the first portion is configured to
acquire data associated with a center region of a k-space associated with
an object to be imaged.

5. The apparatus of claim 4, where the second portion is configured to
acquire data associated with an outer region of the k-space.

6. The apparatus of claim 1, including a trajectory logic to control a
data sampling trajectory performed by an imaging apparatus controlled by
the apparatus.

7. The apparatus of claim 6, the data sampling trajectory being one of,
Cartesian data sampling, and non-Cartesian data sampling.

8. The apparatus of claim 1, where the apparatus provides the combined
acquisition technique pulse sequence to one of, a magnetic resonance
imaging (MRI) apparatus, and a spectroscopy apparatus.

9. The apparatus of claim 1, the second portion being one of, a bSSFP
portion, a FLASH portion, and an incoherent SSFP portion.

10. The apparatus of claim 1, where the apparatus is to provide the
combined acquisition technique pulse sequence to an MRI apparatus, and
where magnetization produced by the MRI apparatus is to approach a steady
state value in the outer regions of k-space.

11. A method, comprising:controlling an imaging apparatus to apply a first
pulse sequence to a subject in a magnetic field produced by the imaging
apparatus, the first pulse sequence being configured to isolate T2
contrast;controlling the imaging apparatus to obtain a first set of echo
signals in response to the application of the first pulse
sequence;controlling the imaging apparatus to apply a second pulse
sequence to the subject, the second pulse sequence being configured to
acquire both T1 and T2 contrast;controlling the imaging apparatus to
obtain a second set of echo signals in response to the application of the
second pulse sequence; andgenerating a combined k-space data set from
first set of echo signals and the second set of echo signals.

12. The method of claim 11, the imaging apparatus being one of, an MR
apparatus, and a spectroscopy apparatus.

13. The method of claim 11, where applying the first pulse sequence
includes encoding a central region of a k-space associated with the
subject, and where acquiring the first set of echo signals includes
acquiring data associated with the central region of the k-space.

14. The method of claim 11, where a point spread function associated with
a k-space data set acquired using a combination of the first pulse
sequence and the second pulse sequence has a first width, and where a
point spread function associated with a k-space data set acquired using
only the first pulse sequence has a second width, where the first width
is less than the second width.

15. The method of claim 11, where a combination of the first pulse
sequence and the second pulse sequence is configured to yield a first RF
deposition when applied for a defined period of time, and where the first
pulse sequence is configured to yield a second RF deposition when applied
for the defined period of time, the first RF deposition being less than
the second RF deposition.

16. The method of claim 11, where a combination of the first pulse
sequence and the second pulse sequence is configured to yield a first
image having a first resolution in a first period of time, and where the
first pulse sequence is configured to yield a second image having a
second resolution in the first period of time, the first resolution being
greater than the second resolution.

17. The method of claim 11, where a first image acquired using a
combination of the first pulse sequence and the second pulse sequence is
to exhibit a first off-resonance property, and where a second image
acquired using only the first pulse sequence is to exhibit a second
off-resonance property, the first off-resonance property being superior
to the second off-resonance property.

18. An MRI apparatus, comprising:at least one radio frequency (RF) coil
configured to generate and receive RF signals;a controller logic
configured to:control the at least one RF coil to generate a first RF
pulse sequence including non-uniformly spaced inversion pulses;control
the at least one RF coil to receive primarily T2 weighted echo signal
data in response to applying the first RF pulse sequence to a subject to
be imaged;control the at least one RF coil to generate a second RF pulse
sequence including no inversion pulses; andcontrol the at least one RF
coil to receive T1 and T2 weighted echo signal data in response to
applying the second RF pulse sequence to the subject to be imaged; anda
memory to store the primarily T2 weighted echo signal data in a k-space
data set and to store the T1 and T2 weighted echo signal data in the
k-space data set.

19. The MRI apparatus of claim 18, where the first RF pulse sequence is to
encode the center of k-space and where the center of the k-space data set
is to store data from the pure T2 weighted echo signal data.

20. The MRI apparatus of claim 19, where magnetization produced by the MRI
apparatus is to approach a steady state value in the outer regions of
k-space.

21. The MRI apparatus of claim 20, where the MRI apparatus performs one
of, a Cartesian data sampling trajectory, and a non-Cartesian data
sampling trajectory.

22. The MRI apparatus of claim 18, where the first RF pulse sequence and
the second RF pulse sequence are controlled to perform three-dimensional
RE-TOSSI.

23. A method, comprising:
TABLE-US-00003
first, repetitively:
{
applying a first steady state radio frequency (RF) pulse sequence to
an area of a subject, the first steady state RF pulse sequence having
three
or more non-uniformly spaced inversion pulses;
obtaining a first echo signal in response to applying the first steady
state RF pulse sequence; and
storing data derived from the first echo signal in a k-space data set;
}, and
second, repetitively:
{
applying a second steady state RF pulse sequence to the area of
the subject, the second steady state RF pulse sequence having no
inversion pulses;
obtaining a second echo signal in response to applying the second
steady state RF pulse sequence; and
storing data derived from the second echo signal in the k-space
data set
}.

24. The method of claim 23, including reconstructing an image from the
k-space data set.

25. The method of claim 23, where the first RF pulse sequence is
configured to produce a signal having a first strength in outer regions
of k-space, where the second RF pulse sequence is configured to produce a
signal having a second strength in outer regions of k-space, and where
the second signal strength is greater than the first signal strength.

26. The method of claim 23, where a point spread function associated with
a k-space data set acquired using a combination of the first RF pulse
sequence and the second RF pulse sequence has a first width, and where a
point spread function associated with a k-space data set acquired using
only the first RF pulse sequence has a second width, where the first
width is less than the second width.

27. A combined acquisition technique, comprising:first controlling an MRI
apparatus to apply a first RF energy to an object for a first period of
time, where the first RF energy is controlled by an initial TOSSI imaging
block; andthen controlling the MRI apparatus to subsequently apply a
second RF energy to the object for a second period of time, where the
second RF energy is controlled by a set of non-inverting, non-TOSSI
acquisitions.

28. The combined acquisition technique of claim 27, where members of the
set of non-inverting, non-TOSSI acquisitions are not identical.

29. The combined acquisition technique of claim 27, where the first period
of time is configurable and where the second period of time is
configurable.

30. The combined acquisition technique of claim 27, where the center of
k-space is to be encoded during the first period of time and where the
outer regions of k-space are to be encoded during the second period of
time.

Description:

PRIORITY CLAIM

[0001]This application claims the benefit of U.S. Provisional Patent
Application 61/000,979 filed Oct. 30, 2007, by the same inventors.

COPYRIGHT NOTICE

[0003]A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The copyright owner has
no objection to the facsimile reproduction of the patent document or the
patent disclosure as it appears in the Patent and Trademark Office patent
file or records, but otherwise reserves all copyright rights whatsoever.

BACKGROUND

[0004]In TOSSI (T1-insensitive Steady State Imaging), unequally spaced
inversion pulses are placed throughout a low flip angle (FA) balanced
steady-state free precession (bSSFP) or TrueFISP (True Fast Imaging With
Steady State Precession) acquisition. These inversion pulses eliminate T1
contrast from bSSFP images of tissues by balancing the signal gained and
lost via T1 relaxation in states parallel and anti-parallel to the main
magnetic field. The resulting signal variations depend on T2 contrast.

SUMMARY

[0005]In Resolution Enhanced TOSSI (RE-TOSSI) the inversion pulses in
TOSSI are eliminated after an adjustable period of time after the center
of k-space is acquired. This results in a TOSSI-non-TOSSI combined
acquisition technique. In RE-TOSSI, magnetization approaches a steady
state value (e.g. bSSFP value) in the outer regions of k-space instead of
decaying toward zero. RE-TOSSI, as compared to TOSSI, provides improved
spatial resolution, faster image acquisition rate, lower radiofrequency
(RF) energy deposition, and improved off-resonance properties in the
outer regions of k-space. For example, experimental results show that
RE-TOSSI provides a 60% faster acquisition than TOSSI. Experimental
results also indicate that RE-TOSSI is 27% faster than a HASTE
(Half-Fourier Acquisition Single-Shot Turbo Spin Echo) acquisition.
Compared to both techniques there is 74% reduction in average RF power
transmission. While the magnitude of the transverse magnetization of
TOSSI decreases throughout the acquisition, the transverse magnetization
of RE-TOSSI remains more constant throughout acquisition. RE-TOSSI images
have improved spatial resolution compared to TOSSI. Thus, RE-TOSSI
provides improvements over conventional acquisition techniques for
certain MRI applications.

[0006]In one example, inversion pulses are removed when the magnetization
is in the aligned state so that the magnetization continually increases
instead of being brought to zero. This facilitates producing a more
constant signal in k-space leading to a narrower point spread function
and having more signal to encode the outer regions of k-space that are
associated with providing resolution. The adjustable parameter λ
determines at what point the TOSSI inversions are stopped and also
modifies the decay curves and point spread functions. Therefore, example
RE-TOSSI systems and methods may reduce the width of the point spread
function (PSF) to where it approaches a near ideal PSF.

MRI Basics

[0007]Magnetic resonance imaging (MRI) is a diagnostic imaging technique
in which image contrasts may be generated. MRI uses a strong magnetic
field to polarize a sample. Different image contrasts may be sensitive to
different pathologies. Two image contrasts for diagnostic medicine are T1
contrast and T2 contrast. T1 (longitudinal) contrast corresponds to the
rate at which magnetization develops parallel to the main magnetic field.
T2 transverse contrast corresponds to the rate at which magnetization
disappears perpendicular to the main magnetic field.

[0008]T2 contrast is often used to facilitate identification of tumors,
tissue affected by stroke and other lesions, pathologies, and other
differences between normal and diseased tissue. Thus, T2 contrast is
useful in clinical imaging. Typically, T2 contrast is generated using a
spin-echo (SE) or turbo-spin-echo (TSE) sequence. A single spin-echo is
formed through a combination of a 90 degree pulse and a 180 degree pulse.
The 90 degree pulse generates signal perpendicular to the main magnetic
field and is followed by the 180 degree pulse that refocuses transverse
magnetization to form an echo. A purely T2 weighted spin echo signal may
be acquired at a point in time after the 180 degree pulse. The time at
which the echo is acquired, the echo time (TE), occurs at a time after
the 180 degree pulse that is equal to the spacing between the 90 degree
pulse and the 180 degree pulse. While a "purely T2 weighted" signal is
described, it will be appreciated by one skilled in the art that MR
images may combine proton density, T1 weighting, and T2 weighting. Thus,
"purely T2 weighted" refers to imaging with reduced and/or minimized T1
effects, and in which the differences in signal levels that exist due to
T2 variations between tissues are also much greater than those due to
proton density differences.

[0009]Single spin-echo imaging sequences that produce pure T2 contrast may
be too slow for certain applications. In some cases, up to 5-10 minutes
may be required to acquire images using single spin-echo imaging. As a
result, pure T2 contrast sequences may have limited use, or be difficult
to apply, in clinical MRI. Thus, turbo or multiple spin-echo sequences
may be employed. These sequences typically use multiple, equally spaced
180 degree pulses after the first 90 degree pulse to generate more than
one spin echo signal. While a 180 degree pulse is described, it is to be
appreciated that some sequences may employ a tip angle of less than 180
degrees to reduce RF energy deposition while maintaining tissue contrast.
A sample turbo spin echo sequence is illustrated in FIG. 1 at 110. Note
that an initial 90 degree pulse is omitted in some sequences illustrated
in FIG. 1. Sequence 110 may facilitate faster acquisition than a single
spin-echo imaging sequence. However, if the 180 degree pulses are
imperfect, unwanted T1 weighting may be generated in the images. The T1
weighting may result from magnetization that has relaxed parallel to the
main magnetic field during the acquisition of the image. In addition,
sequence 110 may require undesirably high depositions of radio-frequency
(RF) power in the patient that may undesirably heat the patient.
Additionally, RF deposition may be exacerbated in apparatus employing
high magnetic field strengths.

[0010]FIG. 1 also illustrates a sequence 120 illustrating T2-prepared
imaging. This type of imaging includes a preparation phase to generate T2
contrast. The preparation phase may be followed by another type of
imaging sequence (e.g., gradient-echo sequence). T2-prepared imaging has
conventionally been limited to cardiac imaging to generate contrast
between blood vessels and cardiac tissue. T2-prepared imaging may see
limited use because T2 contrast fades as an imaging sequence proceeds.
Thus, this method is not broadly used as image intensity depends on
underlying T1 relaxation, which is itself dependent on the actual imaging
sequence used.

[0011]Another conventional technique includes using hyper-echoes to
generate T2 weighted images. This technique includes applying a pattern
of arbitrary pulse angles and phases followed by a 180 degree pulse and
then applying the same pattern of pulses in reverse. This may produce a
single T2 weighted echo. This echo may be referred to as a hyper-echo.
While these sequences may be used to regain T2 weighting after a long
series of pulses, only the hyper-echoes are ensured to be T2 weighted,
while other echoes in the train can have both T1 and T2 weighting.

[0012]The above-described methods share a common approach where an initial
pulse (e.g., 90 degrees) is used to generate transverse magnetization
that is then refocused using a second pulse(s) (e.g., 180 degree, near
180 degree) or other pulses for hyper-echo. Signals resulting from T1
relaxation are avoided or minimized and images with primarily T2 contrast
are generated. However, when these pulses are imperfect, which is common,
the techniques fail to generate pure T2 weighted images and instead
produce a combination of T1 and T2 contrasts.

TOSSI Basics

[0013]One conventional method for reducing undesired T1 contrast in the
detected signals of an MRI sequence included generating pure T2 weighted
images. The method involved allowing signal from T1 relaxation to enter
an image signal but cancelled contrast resulting from different T1
relaxation rates. The method yielded pure T2 weighting in the images. The
method involved applying a train of non-equally spaced 180 degree pulses
that alternatively flipped the longitudinal magnetization into parallel
and anti-parallel states. This method has been referred to as
T1-insensitive Steady State Imaging (TOSSI). Thus, most generally, a
TOSSI sequence included at least three inversion pulses where the spacing
between the two pairs of pulses was not equal. When the timing of the
pulses was chosen appropriately, a constant, greater than zero, level of
longitudinal magnetization may have been generated regardless of the
underlying T1 relaxation rates of the various tissues. Additionally, when
an imaging sequence was implemented in between the 180 degree pulses,
images with pure T2 contrast may have been generated. Sample TOSSI
sequences are illustrated in sequence 130 and sequence 140.

[0014]TOSSI shares some properties with but is fundamentally different
from a technique that employs a series of equally spaced 180 degree
pulses to maintain a constant level of zero longitudinal magnetization.
The equally spaced technique may be used as an image preparation scheme
where no imaging is performed in between the 180 degree pulses. TOSSI
also shares some properties with but is fundamentally different from a
Dixon technique that employs multiple non-equally spaced pulses as a
preparation scheme to generate magnetization for fixed time points in the
imaging sequence so that two types of images can be formed. A first image
concerns fat plus water while a second image concerns fat minus water.
The Dixon technique highlights the T1 contrast for angiographic
applications and does not cancel the effects of T1 relaxation.

[0015]FIG. 2 illustrates an example TOSSI sequence 200. Sequence 200
includes an initial 180 pulse 210, an imaging portion 220, an inversion
pulse 230, another imaging portion 240, another inversion pulse 250, and
another imaging portion 260. In sequence 200, the inversion pulses 230
and 250 are not exactly 180 degree pulses. It is to be appreciated that
inverting pulses need not be exactly 180 degree pulses and thus may be
less than 180 degrees. It will also be appreciated that the 180 degree
pulses can be implemented at places in an imaging sequence where
transverse magnetization is not present. Using this placement in
combination with appropriate spoiling, the 180 degree pulses will not
generate spin-echoes as is the case in the spin and turbo spin echo
sequences. In TOSSI, the 180 degree pulses flip magnetization from
parallel to anti-parallel states to cancel the T1 contrast. Unwanted
transverse magnetization is spoiled using magnetic field gradients.

[0016]In one embodiment, TOSSI may be implemented with a TrueFISP imaging
sequence. While a TrueFISP sequence is described, it is to be appreciated
that other non-TOSSI, non-inverting sequences may be employed. The signal
associated with TrueFISP imaging has a complicated relationship depending
on TE, TR, tip angle, tissue T1, tissue T2, field inhomogeneity, and so
on. The evolution of signal from different tissues with different T1s and
T2s is shown in FIG. 3. Using this approach, a line of k-space data may
be acquired in approximately 5 ms.

[0017]FIG. 3 illustrates a graph 300 of several sets of different T2s
(310-350). Different lines sharing the same identifier (e.g., the three
lines identified as 310) correspond to different T1s. If a sequence was
purely T2 weighted (e.g., primarily T2 weighted) then all lines sharing
an identifier would overlap exactly. However, since the TrueFISP sequence
is affected by T1, different tissues with different T1s show different
signal levels at different parts of the imaging sequence, resulting in
the T2/T1 contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate various example systems, methods,
and other example embodiments of various aspects of the invention. It
will be appreciated that the illustrated element boundaries (e.g., boxes,
groups of boxes, or other shapes) in the figures represent one example of
the boundaries. One of ordinary skill in the art will appreciate that in
some examples one element may be designed as multiple elements or that
multiple elements may be designed as one element. In some examples, an
element shown as an internal component of another element may be
implemented as an external component and vice versa. Furthermore,
elements may not be drawn to scale.

[0044]The following includes definitions of selected terms employed
herein. The definitions include various examples and/or forms of
components that fall within the scope of a term and that may be used for
implementation. The examples are not intended to be limiting. Both
singular and plural forms of terms may be within the definitions.

[0045]References to "one embodiment", "an embodiment", "one example", "an
example", and so on, indicate that the embodiment(s) or example(s) so
described may include a particular feature, structure, characteristic,
property, element, or limitation, but that not every embodiment or
example necessarily includes that particular feature, structure,
characteristic, property, element or limitation. Furthermore, repeated
use of the phrase "in one embodiment" does not necessarily refer to the
same embodiment, though it may.

[0046]ASIC: application specific integrated circuit.

[0047]CD: compact disk.

[0048]CD-R: CD recordable.

[0049]CD-RW: CD rewriteable.

[0050]DVD: digital versatile disk and/or digital video disk.

[0051]HTTP: hypertext transfer protocol.

[0052]LAN: local area network.

[0053]PCI: peripheral component interconnect.

[0054]PCIE: PCI express.

[0055]RAM: random access memory.

[0056]DRAM: dynamic RAM.

[0057]SRAM: synchronous RAM.

[0058]ROM: read only memory.

[0059]PROM: programmable ROM.

[0060]USB: universal serial bus.

[0061]WAN: wide area network.

[0062]"Computer component", as used herein, refers to a computer-related
entity (e.g., hardware, firmware, software, software in execution,
combinations thereof). Computer components may include, for example, a
process running on a processor, a processor, an object, an executable, a
thread of execution, a program, and a computer. A computer component(s)
may reside within a process and/or thread. A computer component may be
localized on one computer and/or may be distributed between multiple
computers.

[0064]"Data store", as used herein, refers to a physical and/or logical
entity that can store data. A data store may be, for example, a database,
a table, a file, a list, a queue, a heap, a memory, a register, a disk,
and so on. In different examples a data store may reside in one logical
and/or physical entity and/or may be distributed between multiple logical
and/or physical entities.

[0065]"Logic", as used herein, includes but is not limited to hardware,
firmware, software and/or combinations thereof to perform a function(s)
or an action(s), and/or to cause a function or action from another logic,
method, and/or system. Logic may include a software controlled
microprocessor, discrete logic (e.g., application specific integrated
circuit (ASIC)), an analog circuit, a digital circuit, a programmed logic
device, a memory device containing instructions, and so on. Logic may
include a gate(s), a combinations of gates, other circuit components, and
so on. In some examples, logic may be fully embodied as software. Where
multiple logical logics are described, it may be possible in some
examples to incorporate the multiple logical logics into one physical
logic. Similarly, where a single logical logic is described, it may be
possible in some examples to distribute that single logical logic between
multiple physical logics.

[0066]An "operable connection", or a connection by which entities are
"operably connected", is one in which signals, physical communications,
and/or logical communications may be sent and/or received. An operable
connection may include a physical interface, an electrical interface,
and/or a data interface. An operable connection may include differing
combinations of interfaces and/or connections sufficient to allow
operable control. For example, two entities can be operably connected to
communicate signals to each other directly or through one or more
intermediate entities (e.g., processor, operating system, logic,
software). Logical and/or physical communication channels can be used to
create an operable connection.

[0067]"Software", as used herein, includes but is not limited to, one or
more computer instructions and/or processor instructions that can be
read, interpreted, compiled, and/or executed by a computer and/or
processor. Software causes a computer, processor, or other electronic
device to perform functions, actions and/or behave in a desired manner.
Software may be embodied in various forms including routines, algorithms,
modules, methods, threads, and/or programs. In different examples
software may be embodied in separate applications and/or code from
dynamically linked libraries. In different examples, software may be
implemented in executable and/or loadable forms including, but not
limited to, a stand-alone program, an object, a function (local and/or
remote), a servelet, an applet, instructions stored in a memory, part of
an operating system, and so on. In different examples, computer-readable
and/or executable instructions may be located in one logic and/or
distributed between multiple communicating, co-operating, and/or parallel
processing logics and thus may be loaded and/or executed in serial,
parallel, massively parallel and other manners.

[0068]Suitable software for implementing various components of example
systems and methods described herein may be developed using programming
languages and tools (e.g., Java, C, C#, C++, SQL, APIs, SDKs, assembler).
Software, whether an entire system or a component of a system, may be
embodied as an article of manufacture and maintained or provided as part
of a machine-readable medium.

[0069]"User", as used herein, includes but is not limited to, one or more
persons, software, computers or other devices, or combinations of these.

DETAILED DESCRIPTION

[0070]A TOSSI sequence of 180 degree pulses with TrueFISP imaging in
between the pulses yields the relaxation illustrated in FIG. 4. In this
example, the magnetization is in the parallel state for 60 ms followed by
20 ms anti-parallel. For the example illustrated in FIG. 4, magnetization
oscillates around a particular level for the beginning of the sequence.
This results in primarily T2 relaxation during this portion of the
imaging sequence. As in FIG. 3, the multiple line groups correspond to
different T2s (410-450), and the different lines sharing an identifier
correspond to varying T1s.

[0071]FIG. 5 illustrates a turbo-spin-echo image 510 acquired over 3
minutes, a TOSSI image 520 acquired over 2 seconds, and a normal TrueFISP
image 530 acquired over 2 seconds. The normal TrueFISP contrast has been
altered using TOSSI to be similar to that of the T2 weighted
turbo-spin-echo image. The TOSSI contrast may be more accurate because
subcutaneous fat, which has a short T2 value, is artificially bright in
the turbo-spin-echo image 510, while it is correctly dark in the TOSSI
image 520. The subcutaneous fat is also particularly bright in the
TrueFISP image 530 due to the short T1.

[0072]In different examples TOSSI may involve more complex
implementations. For example, an inversion pattern may change throughout
the sequence to achieve pure T2 contrast through the acquisition. The
change may be continual. In TOSSI the different patterns still include
inverting (e.g., 180 degree, near 180 degree) pulses. FIG. 6 illustrates
a graph 600 of several sets of T2s (610-650). The beginnings of the
graphed items are identical to those examples illustrated in FIG. 4 for
the first ˜500 ms, but then differ in that they are followed by a
pattern of 30 ms parallel and 20 ms anti-parallel. In this example a more
pure T2 contrast is observed throughout the acquisition. Recall that
TOSSI, described most generally, involves a pulse sequence having at
least three inversion pulses, where the spacing between the inversion
pulses is not equal.

[0073]Primarily T2-weighted contrast has proven valuable for the diagnosis
of pathology using MRI. However, clinical T2-weighted contrast may have
been slow and/or suffered from spatial blurring. TOSSI provided
improvements over conventional pulse sequences due to its primarily T2
weighted contrast and prolonged signal decay compared to HASTE. For
example, TOSSI produced T2 contrast in the head in short imaging times.
However, TOSSI abdominal images may have been blurred due to the shorter
T2 values found in the abdomen. Thus, this application describes systems
and methods associated with RE-TOSSI that address some of the limitations
of TOSSI.

[0074]T2 contrast provides excellent differentiation between normal and
pathologic tissue. Yet T2 contrast techniques may have yielded suboptimal
results. Therefore several hybrid imaging techniques have attempted to
improve spatial resolution and accelerate critical applications. For
example, a combined acquisition technique (CAT) applies distinct imaging
modules in succession to exploit the favorable qualities of the different
techniques in different regions of k-space. CAT acquisitions have been
described that either use different imaging sequences or the same
sequence with different parameters (e.g., bandwidth, flip angle).

[0075]TOSSI has been shown to generate multi-slice T2-weighted images in
the head at 1-2 seconds per image. However, TOSSI's use in the body
suffers from low spatial resolution in the single-shot acquisition. Thus,
this application describes systems and methods associated with RE-TOSSI
that improve the spatial resolution, off-resonance properties, RF energy
deposition, and image acquisition time when compared to TOSSI. The
example systems and methods achieve these improvements while maintaining
the desired T2-weighted contrast. Additionally, in regions of high
magnetic field homogeneity, fat appears hypointense in TOSSI and RE-TOSSI
images (compared to TSE, HASTE and TrueFISP images), which decreases the
need for additional fat suppression.

[0076]A RE-TOSSI sequence includes a first TOSSI portion followed by a
second non-TOSSI, non-inverting portion. The TOSSI portion includes at
least three inversion pulses separated by unequal spaces. The second
portion may be, for example, a True-FISP portion. The decay rate
λ1 (not to be confused with λ of RE-TOSSI described
below) of the magnetization during the transient phase of the balanced
SSFP sequence for on-resonance spins is given by the following equation:

λ 1 = E 1 cos 2 ( α 2 ) + E 2
sin 2 ( α 2 ) [ 1 ] ##EQU00001##

where E1,2=e-TR/T1,2 and α is the flip angle. This
decay rate is a mixture of pure T1 decay (at low flip angles) and pure T2
decay (at flip angles near 180°). The following equation for the
apparent relaxation time T1 * can be derived from equation 1 assuming
TR<<T1, T2:

If T1 relaxation is eliminated (e.g., T1→∞), then the
expression for T1* reduces to:

T 1 * = [ 1 T 2 sin 2 ( α 2 ) ] - 1
[ 3 ] ##EQU00003##

Without T1 relaxation, the equation for the transverse magnetization
Mxy as a function of time is given by:

M xy ( t ) = M 0 sin ( α 2 ) - t T
2 sin 2 ( α 2 ) [ 4 ] ##EQU00004##

While the equation for the z magnetization as a function of time is:

M z ( t ) = M 0 cos ( α 2 ) - t T
2 sin 2 ( α 2 ) [ 5 ] ##EQU00005##

[0077]In TOSSI, T1 contrast is eliminated for a given T2 species by
spending the appropriate time in states parallel and anti-parallel to the
main magnetic field. When the magnetization is parallel to B0, T1
relaxation increases the magnitude of the magnetization vector by a rate
proportional to [M0-|Mz(t)|]. When the magnetization is
anti-parallel to B0, T1 relaxation reduces the magnitude of the
magnetization by a rate proportional to [M0+|Mz(t)|]. The ratio
of the time spent in the parallel state relative to the anti-parallel
state in order to balance the magnetization gained in the parallel state
with that lost in the anti-parallel state and thereby eliminate T1
contrast is given by the following equation:

[0078]R(t) denotes the time dependent ratio of the time spent in the
parallel state (TP) to time spent in the anti-parallel (TA)
state. In the TOSSI pulse sequence a desired flip angle and T2
optimization factor (T2Opt) is specified and used in conjunction
with equations 5 and 6 in order to determine R(t). TA is determined
by the specified number of readouts per anti-parallel block (reads per
down or rpd) times TR plus the time needed for steady state preparation
and storage as well as spoiling. The time in the parallel state (TP)
is then given by R(t)×TA. The number of readouts in the
parallel state can then be calculated with the addition of some dead time
to the parallel bSSFP block in order to fulfill equation 6.

[0079]FIG. 7 illustrates some aspects of some TOSSI embodiments. Section
710 illustrates RF pulses 712 and phase encoding gradients 714 of an
abbreviated TOSSI pulse sequence. In the figure, PP=prep pulses, TP=time
in parallel state, TA=time in anti-parallel state, and TEeff=effective
echo time. In section 710, adiabatic inversion pulses are used to invert
the magnetization between the parallel and anti-parallel states.
Transverse magnetization that is not realigned with the z axis by the
α/2 pulses is eliminated using spoiling gradients. Section 720
illustrates a two state model for TOSSI. The magnetization is inverted
periodically throughout the sequence by unequally spaced inversion pulses
between two states, one parallel and one anti-parallel to B0.
Section 730 illustrates a TOSSI block diagram. In section 730, Np=number
of bSSFP pulses in the parallel state, and Nap=number of bSSFP pulses in
the anti-parallel state. The cycle repeats until the desired phase
encoding lines in K-space are acquired. The solid boxes represent events
while the bulk magnetization is parallel to B0 while the dotted
boxes represent events while the bulk magnetization is anti-parallel.

[0080]To demonstrate TOSSI signal decay and contrast, magnetization
evolution simulations were performed using Matlab (The MathWorks Inc.,
Natick, Mass.). Pure T2 decay was simulated at 180° as well as at
50° (using equation 4) for the T2 values of tissue found in the
abdomen such as cerebrospinal fluid (CSF), spleen, kidney, fat, liver and
muscle (e.g., T2=500, 100, 75, 60, 56 and 50 msec). Chart 810 is a plot
of signal decay as a function of time when a series of 180° pulses
are used such as in the cases of SE and TSE sequences. Chart 820 shows a
plot of the signal decay for the same T2 species when 50° pulses
are used instead. The magnetization has a lower value compared to the
180° case to begin, however, the decay is much slower. The slower
decay leads to less degradation of spatial resolution when signal is
collected and encoded linearly through K-space. The magnetization
evolution for the same tissues now using both T1 and T2 values
(T1/T2=2400/500, 820/100, 650/75, 250/60, 590/76 and 900/50) was
simulated during a bSSFP acquisition (FA=50°, TR=5.2 msec, 192
steps). Chart 830 shows the resulting signal evolution. As is well known,
the signal value goes to a nonzero constant value (steady state) that is
not strictly a function of T2. The simulations were repeated for the same
tissue values using TOSSI (FA=50°, TR=5.2 msec, T2opt=60
msec, 1 rpd, 192 steps). Chart 840 shows the resulting magnetization
decay. Throughout the TOSSI acquisition, the transverse magnetization for
the different tissues is stratified according to T2 values. The TOSSI
decay curves for the simulated tissues with both T1/T2 values included
are very close to the ideal pure T2 decay curves for the same T2 value
(chart 820). This concordance is the basis of the "pure T2 contrast"
associated with TOSSI.

[0081]Although TOSSI is composed of bSSFP imaging blocks, the
magnetization does not approach a steady state value. Instead the
inversion pulses cause the magnetization to steadily decrease from a
starting value toward zero (chart 840). Despite the prolonged signal
decay in TOSSI, compared to the HASTE-like counterpart (chart 810) there
can be significant magnetization decay in the single-shot case. The decay
may depend on T2 value and acquisition duration. This decay leads to
degradation of spatial resolution in the phase encoding direction. This
decay may become more problematic as the number of phase encoding lines
increases and/or as the T2 value of the tissue decreases.

[0082]Example RE-TOSSI systems and methods remove the inversion pulses
from the TOSSI acquisition when the magnetization is in the aligned state
to reintroduce T1 relaxation effects to allow the magnetization to
approach a steady state value instead of driving the magnetization to
zero. This leads to improved spatial resolution. Section 920 demonstrates
how a partial Fourier acquisition is used with the central region of
k-space encoded first during the TOSSI portion of the acquisition and the
outer region of k-space encoded second after the inversions are stopped
(e.g., during the bSSFP portion of the acquisition). This acquisition
maintains T2 contrast since the data around the center of k-space is
acquired using TOSSI while improving spatial resolution due to the
increased signal in the outer regions of k-space during the parallel
bSSFP portion of the acquisition. The partial Fourier factor can be
adjusted and TOSSI prep pulses can be used to change the effective echo
time (TEeff). This affects the degree of T2 contrast and the
signal-to-noise ratio (SNR). The parameter λ describes the fraction
of the k-space lines collected using TOSSI and has a range of 1 (all
TOSSI) to 0 (all balanced SSFP). Note that λ1 was used above
to refer to the rate of magnetization decay during the transient portion
of an SSFP acquisition while in this portion A is used to describe the
relative fraction of TOSSI to non-TOSSI components of an RE-TOSSI
acquisition in k-space.

[0083]FIG. 9 illustrates some example aspects of RE-TOSSI. Section 910
illustrates an example two state model for RE-TOSSI. During the first
stage of the acquisition the magnetization is inverted between states
parallel and anti-parallel to B0 as in TOSSI. During the second
stage the magnetization remains in the state parallel to B0 allowing
the magnetization to relax to a steady state value. Section 920
illustrates a schematic diagram of an example RE-TOSSI acquisition. The
phase encoding (PE) line is plotted as a function of readout number. The
fraction of readouts using TOSSI is denoted in region 922 by λ
while the remaining fraction of readouts using balanced SSFP is denoted
in region 924 by 1-λ. While balanced SSFP is described, it is to be
appreciated that other non-inverting, non-TOSSI portions may be employed.
This diagram depicts a partial Fourier acquisition where acquisition
starts from slightly negative k-space PE line and progresses linearly to
+kmax.

[0084]Simulations were performed to examine RE-TOSSI with respect to
signal decay, contrast, and spatial resolution. Different abdominal
tissues (CSF, spleen, kidney, fat, liver and muscle;
T1/T2=2400/500, 820/100, 650/75, 250/60, 590/76 and 900/50)
were simulated during TOSSI (FA=50°, TR=5.2 msec, T2opt=60
msec, 1 rpd, 192 steps) and RE-TOSSI (λ=0.3, other parameters same
as TOSSI) acquisitions. A zero-padded (8192 points) Fourier transform of
the transverse magnetization decay curves was performed to obtain the
point spread functions (PSF) of the two techniques for each tissue. A
zero-padded (8192 points) Fourier transform of a constant function was
also calculated to demonstrate an ideal PSF (point spread function) for
comparison.

[0085]FIG. 10 illustrates examples of simulated TOSSI (1010) and RE-TOSSI
(1020) (λ=0.3) signal decay curves for on-resonance spins. Each
line represents a different tissue with a different T1/T2 value (tissue
type, T1/T2 given in 840 figure legend). For the first acquisition
segment (e.g., lines 1-55), both sequences demonstrate T2 weighting as
evidenced by the signal evolution curves segregation by T2 values. Note
the elevated signal in the RE-TOSSI simulation 1020 compared with
traditional TOSSI 1010 during the latter readouts (e.g., lines 55-192)
where the later portion of the RE-TOSSI acquisition 1020 does not include
inversion pulses. In RE-TOSSI, signals approach steady state values
instead of continuing to decay as in traditional TOSSI.

[0086]Chart 1030 illustrates example TOSSI and RE-TOSSI PSF for simulated
kidney tissue. For comparison, the simulated PSF of a tissue with
T2=infinity is also plotted. In this example, the RE-TOSSI PSF has a
full-width at half maximum (FWHM) that is 52.8% narrower than TOSSI.
Additionally, the first and second side lobe amplitudes are decreased by
27.4% and 11.3% respectively. The RE-TOSSI PSF has a FWHM that approaches
the ideal PSF. Differences include side lobe amplitudes that are 49.5%
and 56.9% higher than the ideal/sinc PSF for the first and second side
lobes respectively.

[0087]Experiments in phantoms determined the contrast and spatial
resolution properties of RE-TOSSI. 50 ml syringes were prepared with
agarose with the following weight to volume (wt/vol) agarose content: 0,
0.5, 1.0, 2.0 and 4.0%. The phantoms were placed in the head matrix coil
(for signal reception) inside a 1.5 T Siemens Espree MR scanner
(Erlangen, Germany). The T2 values of the phantoms were measured using a
multi-contrast spin echo sequence (21 echo times between 18-388 msec,
TR=10,000 msec). The T2 value of each phantom was calculated using
fitting software. A multi-slice TSE (TE/TR=72/3000 msec, ETL=19), a
multi-slice slice TrueFISP (FA=50°, TE/TR=2.3/4.7 msec), a
multi-slice slice HASTE (TE/TR=63/3000 msec), a single slice TOSSI
(TEeff=415 msec, FA=50°, TR=5.2 msec, PF=4/8, 1 rpd, 50 prep
pulses, BW=500 Hz/Px) and two different single slice RE-TOSSI
(λ=0.5, λ=0.04) acquisitions were used to acquire images at
the same location within the phantoms. Images had a 230 mm×230 mm
field of view (FOV), 256×256 matrix, and 5 mm slice thickness.

[0088]To quantify the resolution properties of the different sequences,
the edge response functions (ERF) were extracted from the phantom images
and analyzed using established transfer function methods. The ERF were
smoothed using a [1 2 1] kernel and differentiated to generate line
spread functions (LSF). The modulation transfer function (MTF) was
calculated as the normalized magnitude of the Fourier transform of the
LSF. The information transfer function (ITF) was calculated as the square
of the MTF. The cutoff frequency (fc) was determined as the
frequency at which the magnitude of the MTF reached 1/10 of its peak
height. The area under the MTF curve (MTFA) and the equivalent bandpass
(area under the ITF curve) were calculated. The optimal frequency
response, defined as the maximum of the (MTF(k)×k) as a function of
k (spatial frequency) was determined.

[0090]Examining FIG. 11 more closely yields the following results. Image
1110 shows the T2 contrast obtained using TSE. The SE-based estimate of
the T2 value for each vial of the phantom is shown in the image. Note
that the signal intensity drops from left to right as the T2 values get
shorter. Image 1120 shows the results of the TrueFISP imaging. Note the
diminished contrast between the agarose phantoms. Image 1130 shows
results from the HASTE sequence. Note the increased degradation of
spatial resolution along the phase encoding (vertical) direction as the
T2 values decrease. Images 1140 and 1150 show the results of RE-TOSSI
imaging with λ=0.04 and λ=0.49 respectively. Note the visual
similarity in contrast between the RE-TOSSI images and that of the TSE.
Note also the sharp edges of the phantoms in 1140. Image 1160 shows the
result of imaging with TOSSI. Note the T2 contrast but the apparent loss
of resolution at distinct edges. However, this loss of resolution appears
to be less than in the HASTE image. The edge response functions (ERFs)
are plotted in figure 1170. Note the decreased slope of the TOSSI and
HASTE ERFs compared to TSE and bSSFP. The RE-TOSSI (λ=0.04) ERF has
a higher slope than the TOSSI ERF. The modulation transfer functions for
the six imaging techniques are presented in figure 1180. Note the widened
MTF of RE-TOSSI (λ=0.04) compared to TOSSI indicating improved
spatial resolution characteristics of the RE-TOSSI image compared to the
TOSSI image. A quantitative comparison of the MTFs for the various
techniques are presented in table 1.

[0091]Imaging of an asymptomatic human volunteer was performed to assess
the in-vivo abdominal imaging capability of RE-TOSSI. An asymptomatic
human volunteer was placed head first, supine in the scanner bore. The
body matrix coil was placed on the lower anterior thorax and abdomen of
the volunteer. Transverse images at the same locations were acquired
using a multi-slice TSE (TE/TR=60/3000, ETL=15), a multi-slice TrueFISP
(FA=50°, TE/TR=2.1/4.2 msec), a multi-slice HASTE (TE/TR=62/3000
msec), a multi-slice traditional TOSSI (TEeff=85 msec,
FA=50°, TR=5.2 msec, T2opt=90, half Fourier, 1 rpd, 1 prep
pulse, BW=500) and a multi-slice RE-TOSSI (λ=0.2, TEeff=96
msec, 3 prep pulses). The images were acquired with a 320 mm×240 mm
FOV, 256×192 matrix and 5 mm slice thickness. The body matrix coil
and the spine matrix coil in the table under the volunteer were used for
signal reception. The acquisition time and RF power deposition for each
scan as reported by the scanner were recorded.

[0092]FIG. 12 compares sequences by illustrating images acquired in the
abdomen of a healthy human volunteer in the transverse plane. Image 1210
illustrates fat-saturated T2W TSE. Note the T2 contrast and distinct
edges suggesting a limited loss of resolution. Note also the presence of
breathing artifacts since the volunteer could not hold their breath for
the entire acquisition time and the incomplete fat saturation at the
anterior abdominal wall. Image 1220 illustrates the HASTE imaging result.
Note the apparent loss in resolution throughout the image, with the loss
being worse in muscle and fat. Note also the hyperintense fat signal and
the hypointense signal compared to TSE in the liver and pancreas. Image
1230 shows the RE-TOSSI images with λ=0.2. Note the similarity in
contrast between the RE-TOSSI image 1230 and the TSE image 1210. The
similarity in resolution between the images suggests an acceptable point
spread function and the homogeneously hypointense signal from fat. Image
1240 shows a TrueFISP image. Note the hyperintense fat signal as well as
limited loss of resolution in the steady state image. Image 1250 shows a
traditional TOSSI image of the same location. Note the T2 contrast but
severe degradation of spatial resolution.

[0093]Table 2 shows a comparison of image acquisition time and average RF
energy deposition for the images shown in FIG. 12. RE-TOSSI provides a
60% faster acquisition than TOSSI. RE-TOSSI is also 27% faster than the
HASTE acquisition. Compared to both techniques there is 74% reduction in
average RF power transmission.

[0094]To determine and compare the off-resonance properties of examples of
TOSSI and RE-TOSSI (λ=0.3), the Bloch equation simulations were
repeated using the following parameters: TR=5 msec, FA=50°,
T2opt=60 msec, 1 rpd and 140 readouts. The T1/T2 of the simulated spins
were 250/60 msec. -2π to 2π radians (in steps 0.1 radian)
off-resonance precession per TR was included. For comparison, simulations
of bSSFP with the same off-resonance precession angles were performed
using the same TR, flip angle, number of readouts and T1/T2 values.

[0095]FIG. 13 illustrates the results of off-resonance simulations. Images
1310-1330 demonstrate the off-resonance properties of example bSSFP,
TOSSI, and RE-TOSSI respectively throughout the acquisition of 140
readouts. The images 1310 through 1330 illustrate the log of the
transverse magnetization plotted as a function of off-resonance angle and
readout number. Note the characteristic banding pattern of bSSFP in image
1310 and the widened bands of TOSSI in image 1320. In image 1330, note
the similar bands of RE-TOSSI compared to TOSSI during the beginning of
the acquisition (e.g., readouts 1-50) and the significantly narrower
bands after the inversion pulses have been stopped (e.g., readouts
51-140).

[0096]In order to demonstrate the off-resonance properties in more detail,
the transverse magnetization was plotted as a function of off-resonance
angle for readout numbers 45, 72, and 120. These readout numbers
correspond to signal levels during the initial 1340, transition 1350, and
final 1360 portions of the RE-TOSSI acquisition. The off-resonance
profiles for the three acquisitions during readout number 45 are plotted
in image 1340. The FWHM of the off-resonance bands are 1.2 rad for bSSFP
while TOSSI and RE-TOSSI have identical off-resonance profiles with a
FWHM of 3.4 rad, which is 2.8 times wider. After the inversions are
stopped, the RE-TOSSI off-resonance profiles begin to diverge from those
of TOSSI. The off-resonance profiles are plotted for readout number 72 in
image 1350. Note that the FWHM for the bSSFP and RE-TOSSI are both 1.1
rad while that of TOSSI has increased to 4.8 rad, which is 4.4 times
wider. The off-resonance profiles are plotted again for one of the later
readouts (e.g., number 120). The RE-TOSSI FWHM is the same as that for
bSSFP (1.0 rad) while the TOSSI FWHM is 4.9 rad, which is 4.9 times
wider. Note that the magnitude of the transverse magnetization of TOSSI
decreases throughout the acquisition, while that of RE-TOSSI remains
fairly constant (around 0.2).

[0097]To investigate the nature and extent of the off-resonance banding
properties of examples of TOSSI and RE-TOSSI in vivo, the bSSFP, TOSSI
and RE-TOSSI sequences were used to acquire 350 mm×350 mm FOV
images (256×256 matrix) with 5 mm slice thickness in the coronal
plane in the same short bore magnet in a different asymptomatic
volunteer. FIG. 14 illustrates coronal images acquired from an
asymptomatic volunteer thorax and abdomen and demonstrates the in-vivo
off-resonance banding properties of bSSFP 1410, TOSSI 1420, and RE-TOSSI
(λ=0.2) 1430. Note the characteristic banding pattern of bSSFP
(figure 1410) as the field homogeneity decreases toward the ends of the
magnet as illustrated at the top and bottom of the image. Note that the
bands for TOSSI and RE-TOSSI appear similar in size and location to each
other. They also occur in the same places as those of bSSFP but are wider
in size. As was also demonstrated in the transverse plane images, the
RE-TOSSI image has improved spatial resolution compared to TOSSI.

[0098]The on-resonance simulations illustrated in FIG. 10 show that the
loss in spatial resolution associated with single-shot TOSSI images in
the abdomen (image 1250 (FIG. 12) and image 1420 (FIG. 14)) is due to a
lack of signal in the outer regions of k-space (image 1010 (FIG. 10)).
The decay of the transverse magnetization during data acquisition leads
to a widened point-spread function (image 1030 (FIG. 10)) which is
convolved with the underlying anatomical information producing the
degradation of spatial resolution in the phase encoding direction. The
PSF is tissue dependent due to differences in T1 and T2 across tissues.
During TOSSI the signal from each tissue oscillates around the prolonged
pure T2 decay curve at low flip angles given by equation 4. In one
example of RE-TOSSI, the inversion pulses are removed from the TOSSI
pulse sequence after acquiring the center of k-space, the region
associated with generating contrast. This allows T1 relaxation effects to
restore the magnetization to the balanced SSFP value (image 1020 (FIG.
10)) instead of forcing the magnetization to continue to decay via the
conventional TOSSI pattern (image 840 (FIG. 8), image 1010 (FIG. 10)). In
one example, inversion pulses are removed when the magnetization is in
the aligned state so that the magnetization continually increases instead
of being brought to zero. This facilitates producing a more constant
signal in k-space leading to a narrower point spread function (image 1030
(FIG. 10)) and having more signal to encode the outer regions of k-space
that are associated with providing resolution (image 1020 (FIG. 10)). The
adjustable parameter λ determines at what point the inversions are
stopped and also modifies the decay curves and point spread functions.
Therefore, example RE-TOSSI systems and methods may reduce the width of
the point spread function to where it approaches a near ideal PSF (image
1030 (FIG. 10)).

[0099]The agarose phantom results illustrated in FIG. 11 highlight some of
the limitations of some conventional imaging techniques. These
limitations include lack of T2 contrast (image 1120) and degradation of
spatial resolution in the single-shot techniques for short T2 species
(image 1130), as often found in abdominal imaging. Example RE-TOSSI
systems and methods are capable of maintaining the T2 contrast obtained
using TOSSI (image 1160) while improving high resolution information
(e.g., edge conspicuity (image 1140)). The preparatory pulses in the
TOSSI and RE-TOSSI sequences prior to readout were used to establish the
T2 contrast in the partial Fourier acquisition. Because of the prolonged
decay at the lower flip angles, a longer effective echo time is needed
for similar contrast. In this TOSSI technique, magnetization does not
decay as quickly and therefore the single shot TOSSI images (image 1160)
are less blurry than the HASTE images (image 1130). Alternatively,
instead of using preparatory pulses, initial TOSSI pulses may encode some
of the more negative lines of k-space that were skipped in the partial
Fourier acquisition. The RE-TOSSI images demonstrate that T2-weighting
can be maintained even for very small values of λ (image 1140).
These results highlight the role of the center of k-space in determining
image contrast. The edge response functions (image 1170) demonstrate the
loss in spatial resolution associated with the different techniques.
Quantitative analysis by means of the modulation transfer function as
illustrated in image 1180 and table 1 demonstrate that example RE-TOSSI
systems and methods improve the resolution properties of TOSSI to near
those of TSE and bSSFP.

[0100]The human abdominal imaging results (image 1230 (FIG. 12), image
1430 (FIG. 14)) demonstrate the in vivo potential of generating fast
T2-weighted images using RE-TOSSI. RE-TOSSI reduced the apparent loss in
spatial resolution seen in the traditional TOSSI image (image 1250 (FIG.
12), image 1420 (FIG. 14)) and produced an in-vivo image with similar
contrast to the fat-suppressed T2-weighted TSE (image 1210 (FIG. 12)).
The excellent field homogeneity in the transverse plane allowed the
acquisition of TOSSI (image 1230 (FIG. 12)) and RE-TOSSI images (image
1250 (FIG. 12)) of the human abdomen free from major banding artifacts
despite the widened regions of off-resonance signal loss associated with
TOSSI as illustrated in image 1340 (FIG. 13). The low fat signal in image
1230 (FIG. 12) and image 1250 (FIG. 12) is due, at least in part, to the
low T2 value of fat. This low T2 value of fat leads to a low intrinsic
signal value in TOSSI as illustrated in 840 (FIG. 8). There is a degree
of additional TR and magnetic field homogeneity dependent suppression of
fat caused by the off-resonance properties of TOSSI. The intrinsic
hypointense fat signal is an attractive aspect of TOSSI since no
additional time needs to be spent on fat saturation. No additional time
is spent given that the other methods of generating fast T2 contrast
produce a hyperintense fat signal (e.g., image 1220 (FIG. 12)) and fat
suppression methods can be prone to failure as illustrated in image 1210
(FIG. 12) of the anterior abdominal wall.

[0101]In RE-TOSSI, there is a reduction in imaging time as compared to
TOSSI due to the removal of inversion pulses and the associated steady
state storage and preparation pulses and spoiling gradients. Assuming a
patient breath hold of 22 seconds, the current image 2D acquisition time
of 0.76 sec/image would allow ˜29 slices to be acquired in a single
breath hold. For 5 mm slices this corresponds to 14 cm of continuous
coverage, corresponding to the average size of the liver at the
midclavicular line. With a parallel imaging acceleration factor of 2, by
introducing a slight gap between the slices or by increasing the slice
thickness to 8 mm, it will be possible to cover the liver of most
patients in a single breath hold.

[0102]RE-TOSSI produces a reduction in the average energy deposition
compared to TOSSI as illustrated in table 2. In one example the reduction
achieved 74%. This decrease in average energy deposition is caused by the
elimination of energy intensive adiabatic inversion pulses. As
implemented, some TOSSI examples approached the higher end of the allowed
specific absorption rate (SAR) values (˜90% of allowed). RE-TOSSI
demonstrates one of the lower SAR values (˜25% of allowed), which
is only slightly above that of bSSFP. Since SAR scales with the square of
the Larmor frequency, the fourfold decrease in SAR will make it possible,
from an RF energy deposition perspective, to use the RE-TOSSI sequence at
higher fields (e.g., >1.5 T) for the fast generation of T2 contrast
without further modifications.

[0103]Off-resonance simulations as illustrated in FIG. 13 have shown that
TOSSI as currently implemented with α/2 preparation and storage
pulses has widened areas of low signal as a function of off-resonance
angle throughout k-space compared to bSSFP. This signal behavior
corresponds to wider bands in TOSSI images (image 1420 (FIG. 14)) in
regions of inhomogeneous magnetic field than seen in conventional bSSFP
acquisitions as illustrated in image 1410 (FIG. 14). Signal reduction
occurs when magnetic field inhomogeneity exceeds 0.75 ppm. Example
RE-TOSSI acquisition techniques demonstrate better off-resonance
properties than TOSSI as illustrated in image 1320 (FIG. 13) and image
1330 (FIG. 13). During the TOSSI portion of a RE-TOSSI acquisition, the
off-resonance properties are the same as illustrated in image 1340 (FIG.
13). However, after the inversion pulses are stopped, the off-resonance
band widths begin to decrease as illustrated in image 1350 (FIG. 13). The
off-resonance band widths go back to the more narrow bSSFP case in the
later portion of the acquisition as illustrated in 1160 (FIG. 11). The
width of the bands in image 1420 (FIG. 14) and image 1430 (FIG. 14) will
be essentially equivalent since they are determined by contrast at the
center of k-space. There is high resolution information that is gained in
RE-TOSSI in the regions where the low signal become narrower compared to
the TOSSI acquisition as illustrated in image 1350 (FIG. 13) and image
1360 (FIG. 13). This corresponds to less ghosting and a gain in
resolution for some off-resonant species. The widened bands illustrated
in image 1340 (FIG. 13) during the TOSSI portion of the acquisition that
occur around the center of k-space further suppress the fat signal beyond
the already intrinsically low fat signal in TOSSI when an appropriate TR
is used. For example, TRs between 1.1-3.3 milliseconds and 5.6-7.8
milliseconds will suppress the fat signal by a factor of two or greater.
The in-vivo coronal RE-TOSSI image 1430 (FIG. 14) demonstrates that even
in a short bore magnet similar to that used in the studies reported
herein there is a large region (20-25 cm) around the center of the magnet
that is free from the majority of the banding artifacts. This validates
the use of RE-TOSSI.

[0104]Similar to other combined acquisition techniques, RE-TOSSI includes
a parameter λ, which in this case characterizes the fraction of
k-space acquired using TOSSI. Combined acquisition techniques have used
different pulse sequence blocks in various regions of K-space. In
RE-TOSSI, the fundamental repeating imaging block is balanced SSFP with
the same parameters (e.g. same flip angle, bandwidth, etc) throughout the
sequence. RE-TOSSI includes a TOSSI portion and a non-inverting non-TOSSI
portion. There does not appear to be "an instantaneous jump" in signal
amplitude between the inverting and the non-inverting portions. This jump
may not appear when the fundamental repeating units in both portions are
similar. In RE-TOSSI, λ characterizes the pure T2-weighting
throughout k-space in the RE-TOSSI sequence and the magnetization's
approach to the bSSFP steady state value. λ can be chosen so that
the resulting image retains T2-weighting provided by TOSSI while also
improving spatial resolution, imaging time, off-resonance signal loss and
RF energy deposition.

[0105]While Cartesian RE-TOSSI acquisitions have been described and
experimentally verified it is to be appreciated that RE-TOSSI can be used
with non-Cartesian data acquisition techniques. For example, RE-TOSSI can
be used to improve the resolution of TOSSI using annular ring or
concentric ring data acquisition schemes. The rings around the center of
k-space can be acquired using TOSSI and then the outer k-space rings can
be acquired with a non-TOSSI, non-inverting sequence. The signal in the
outer regions of k-space will be increased using RE-TOSSI and similar
improvements in spatial resolution, acquisition time, RF power deposition
and off-resonance properties will be realized compared to a pure TOSSI
sequence.

[0107]FIG. 19 illustrates an example computing device with which example
systems and methods described herein, and equivalents, may interact. The
example computing device may be a computer 1900 that includes a processor
1902, a memory 1904, and input/output ports 1910 operably connected by a
bus 1908. In one example, the computer 1900 may include a RE-TOSSI logic
1930 configured to facilitate producing a RE-TOSSI pulse sequence and
receiving signal data in response to the RE-TOSSI signal. In different
examples, the logic 1930 may be implemented in hardware, software,
firmware, and/or combinations thereof. Thus, the logic 1930 may provide
means (e.g., hardware, software, firmware) for generating a sequence
having two parts, a first pure T2-weighted segment and a second T1-T2
segment, where the T2-weighted segment acquires signal in and around the
center of k-space. While the logic 1930 is illustrated as a hardware
component attached to the bus 1908, it is to be appreciated that in one
example, the logic 1930 could be implemented in the processor 1902.

[0109]A disk 1906 may be operably connected to the computer 1900 via, for
example, an input/output interface (e.g., card, device) 1918 and an
input/output port 1910. The disk 1906 may be, for example, a magnetic
disk drive, a solid state disk drive, a floppy disk drive, a tape drive,
a Zip drive, a flash memory card, and/or a memory stick. Furthermore, the
disk 1906 may be a CD-ROM, a CD recordable drive (CD-R drive), a CD
rewriteable drive (CD-RW drive), and/or a digital video ROM drive (DVD
ROM). The memory 1904 can store a process 1914 and/or a data 1916, for
example. The disk 1906 and/or the memory 1904 can store an operating
system that controls and allocates resources of the computer 1900.

[0110]The bus 1908 may be a single internal bus interconnect architecture
and/or other bus or mesh architectures. While a single bus is
illustrated, it is to be appreciated that the computer 1900 may
communicate with various devices, logics, and peripherals using other
busses (e.g., PCIE, SATA, Infiniband, 1394, USB, Ethernet). The bus 1908
can be types including, for example, a memory bus, a memory controller, a
peripheral bus, an external bus, a crossbar switch, and/or a local bus.

[0111]The computer 1900 may interact with input/output devices via the i/o
interfaces 1918 and the input/output ports 1910. Input/output devices may
be, for example, a keyboard, a microphone, a pointing and selection
device, cameras, video cards, displays, the disk 1906, the network
devices 1920, and so on. The input/output ports 1910 may include, for
example, serial ports, parallel ports, and USB ports.

[0112]The computer 1900 can operate in a network environment and thus may
be connected to the network devices 1920 via the i/o interfaces 1918,
and/or the i/o ports 1910. Through the network devices 1920, the computer
1900 may interact with a network. Through the network, the computer 1900
may be logically connected to remote computers. Networks with which the
computer 1900 may interact include, but are not limited to, a local area
network (LAN), a wide area network (WAN), and other networks.

[0113]FIG. 20 illustrates an example MRI apparatus 2000 that includes a
RE-TOSSI logic 2090. The apparatus 2000 includes a basic field magnet(s)
2010 and a basic field magnet supply 2020. Ideally, the basic field
magnets 2010 would produce a uniform B0 field. However, in practice,
the B0 field may not be uniform, and may vary over an object being
imaged by the MRI apparatus 2000. MRI apparatus 2000 may include gradient
coils 2030 configured to emit gradient magnetic fields like GS,
GP and GR. The gradient coils 2030 may be controlled, at least
in part, by a gradient coils supply 2040.

[0114]MRI apparatus 2000 may also include an RF antenna 2050 that is
configured to generate RF pulses and to receive resulting magnetic
resonance signals from an object to which the RF pulses are directed. In
some examples, how the pulses are generated and how the resulting MR
signals are received may be controlled and thus may be selectively
adapted during an MRI procedure. In one example, separate RF transmission
and reception coils can be employed. The RF antenna 2050 may be
controlled, at least in part, by an RF transmission-reception unit 2060.
The gradient coils supply 2040 and the RF transmission-reception unit
2060 may be controlled, at least in part, by a control computer 2070. In
one example, the control computer 2070 may be programmed to perform
methods like those described herein (e.g., generate RE-TOSSI sequence).

[0115]The magnetic resonance signals received from the RF antenna 2050 can
be employed to generate an image, and thus may be subject to a
transformation process like a two dimensional FFT that generates
pixilated image data. The transformation can be performed by an image
computer 2080 or other similar processing device. The image data may then
be shown on a display 2099. While an MR apparatus 2000 is illustrated, it
is to be appreciated that in some examples RE-TOSSI may be employed with
other imaging apparatus and/or methods (e.g., spectroscopy).

[0116]While FIG. 20 illustrates an example MRI apparatus 2000 that
includes various components connected in various ways, it is to be
appreciated that other MRI apparatus may include other components
connected in other ways. In one example, to implement the example systems
and methods described herein, MRI apparatus 2000 may be configured with a
RE-TOSSI logic 2090. In different examples, RE-TOSSI logic 2090 may be
permanently and/or removably attached to an MRI apparatus. While RE-TOSSI
logic 2090 is illustrated as a single logic connected to control computer
2070 and image computer 2080, it is to be appreciated that RE-TOSSI logic
2090 may be distributed between and/or operably connected to other
elements of apparatus 2000. RE-TOSSI logic 2090 may execute portions of
the methods described herein.

[0117]FIG. 21 illustrates an example apparatus 2100 associated with
RE-TOSSI. Apparatus 2100 comprises a first logic 2110 to produce a TOSSI
pulse sequence having at least three inversion pulses. In one example,
the spacing between the at least three inversion pulses is not equal.
Apparatus 2100 also comprises a second logic 2120 to produce a
non-inverting, non-TOSSI pulse sequence. The non-inverting, non-TOSSI
pulse sequence does not include inversion pulses associated with a TOSSI
pulse sequence. Apparatus 2100 also comprises a combination logic 2130 to
provide a combined acquisition technique pulse sequence. The combined
acquisition technique pulse sequence comprises a first portion provided
by the first logic 2110 and a second portion provided by the second logic
2120. The first portion precedes the second portion. In one example, the
first portion is configured to acquire data associated with a center
region of a k-space associated with an object to be imaged. In the
example, the second portion is configured to acquire data associated with
an outer region of the k-space. In one example, the second portion may be
a bSSFP portion.

[0118]In one example, the at least three inversion pulses are to be
distributed in a low flip angle BSSFP acquisition. In one example, at
least three inversion pulses are configured to balance signal gained and
lost via T1 relaxation in states parallel and anti-parallel to a main
magnetic field produced by an imaging system controlled by the apparatus
2100.

[0119]In one example, apparatus 2100 may also include a trajectory logic
to control a data sampling trajectory performed by an imaging apparatus
controlled by the apparatus 2100. The data sampling trajectory may be,
for example, a Cartesian data sampling trajectory, and a non-Cartesian
data sampling trajectory.

[0120]Apparatus 2100 may provide the combined acquisition technique pulse
to different imaging apparatus. For example, the apparatus 2100 may
provide the combined acquisition technique pulse sequence to a magnetic
resonance imaging (MRI) apparatus and/or and a spectroscopy apparatus. In
one example, the apparatus 2100 provides the combined acquisition
technique pulse sequence to an MRI apparatus and controls the MRI
apparatus so that magnetization produced by the MRI apparatus approaches
a steady state value in the outer regions of k-space.

[0121]Example methods may be better appreciated with reference to flow
diagrams. For purposes of simplicity of explanation, the illustrated
methodologies are shown and described as a series of blocks. However, it
is to be appreciated that the methodologies are not limited by the order
of the blocks, as some blocks can occur in different orders and/or
concurrently with other blocks from that shown and described. Moreover,
less than all the illustrated blocks may be required to implement an
example methodology. Blocks may be combined or separated into multiple
components. Furthermore, additional and/or alternative methodologies can
employ additional, not illustrated blocks.

[0122]FIG. 22 illustrates a method 2200. Method 2200 includes, at 2210,
controlling an imaging apparatus to apply a first pulse sequence to a
subject in a magnetic field produced by the imaging apparatus. The first
pulse sequence is configured to isolate T2 contrast. The imaging
apparatus may be, for example, an MRI apparatus.

[0123]Method 2200 also includes, at 2220, controlling the imaging
apparatus to obtain a first set of echo signals in response to the
application of the first pulse sequence. In one example, applying the
first pulse sequence includes encoding a central region of a k-space
associated with the subject. In the example, acquiring the first set of
echo signals includes acquiring data associated with the central region
of the k-space.

[0124]Method 2200 also includes, at 2230, controlling the imaging
apparatus to apply a second pulse sequence to the subject. The second
pulse sequence is configured to acquire both T1 and T2 contrast.

[0125]Method 2200 also includes, at 2240, controlling the imaging
apparatus to obtain a second set of echo signals in response to the
application of the second pulse sequence.

[0126]Method 2200 also includes, at 2250, generating a combined k-space
data set from first set of echo signals and the second set of echo
signals. In one example, an image may be reconstructed from the combined
k-space data set.

[0127]The combination of the first pulse sequence and the second pulse
sequence produces superior results when compared to only applying the
first pulse sequence. For example, a point spread function associated
with a k-space data set acquired using a combination of the first pulse
sequence and the second pulse sequence may have a first width. However, a
point spread function associated with a k-space data set acquired using
only the first pulse sequence will have a second width, and the first
width will be less than the second width. Similarly, a combination of the
first pulse sequence and the second pulse sequence may yield a first RF
deposition when applied for a defined period of time while applying only
the first pulse sequence for the defined period of time will yield a
second RF deposition. The first RF deposition will be less than the
second RF deposition. The superior results may also be noticed in image
resolution. For example, a combination of the first pulse sequence and
the second pulse sequence may yield a first image having a first
resolution in a first period of time while applying only the first pulse
sequence may yield a second image having a second resolution in the first
period of time. Results indicate that the first resolution will be
greater than the second resolution. The superior results may also be
noticed in off-resonance properties. For example, a first image acquired
using a combination of the first pulse sequence and the second pulse
sequence will exhibit a first off-resonance property while a second image
acquired using only the first pulse sequence will exhibit a second
off-resonance property, Results indicate that the first off-resonance
property will be superior to the second off-resonance property.

[0128]In one example, instructions to control a method may be implemented
as computer executable instructions. Thus, in one example, a
computer-readable medium may store computer executable instructions that
if executed by a machine (e.g., processor) cause the machine to perform a
method. While executable instructions associated with one method are
described as being stored on a computer-readable medium, it is to be
appreciated that executable instructions associated with other example
methods described herein may also be stored on a computer-readable
medium.

[0129]FIG. 23 illustrates an MRI apparatus 2300 associated with RE-TOSSI.
Apparatus 2300 includes at least one radio frequency (RF) coil 2310
configured to generate and receive RF signals. Apparatus 2300 also
includes a controller logic 2320. The controller logic 2320 will control
the RF coil 2310 to generate a first RF pulse sequence that includes
non-uniformly spaced inversion pulses. The controller logic 2320 will
also control the RF coil 2310 to receive primarily T2 weighted echo
signal data in response to applying the first RF pulse sequence to a
subject to be imaged. The controller logic 2320 will also control the RF
coil 2310 to generate a second RF pulse sequence including no inversion
pulses. The controller logic 2320 will also control the RF coil 2310 to
receive T1 and T2 weighted echo signal data in response to applying the
second RF pulse sequence to the subject to be imaged.

[0130]Apparatus 2300 also includes a memory 2330. Memory 2330 will store
the primarily T2 weighted echo signal data in a k-space data set and also
store the T1 and T2 weighted echo signal data in the k-space data set. In
one example, the first RF pulse sequence is to encode the center of
k-space. Therefore, the center of the k-space data set is to store data
from the pure T2 weighted echo signal data.

[0131]In one example, magnetization produced by the MRI apparatus 2300 is
to approach a steady state value in the outer regions of k-space. The MRI
apparatus 2300 may perform different types of sampling. For example, the
MRI apparatus 2300 may perform a Cartesian data sampling trajectory,
and/or a non-Cartesian data sampling trajectory. One skilled in the art
will appreciate that controller logic 2320 may control the MRI apparatus
2300 to perform two dimensional and/or three dimensional imaging.
Therefore, the first RF pulse sequence and the second RF pulse sequence
may be controlled to perform three-dimensional RE-TOSSI.

[0132]FIG. 24 illustrates a method 2400 associated with RE-TOSSI. Method
2400 includes two loops where actions may be performed repetitively. In
the first loop, method 2400 includes, at 2410, applying a first steady
state RF pulse sequence to an area of a subject. The first steady state
RF pulse sequence has three or more non-uniformly spaced inversion
pulses. Method 2400 also includes, at 2420, obtaining a first echo signal
in response to applying the first steady state RF pulse sequence at 2410.
Method 2400 also includes, at 2430, storing data derived from the first
echo signal in a k-space data set. This inverting (e.g., TOSSI) portion
of a pulse sequence may be applied one or more times. Therefore, method
2400 includes, at 2440, determining whether to repeat the set of actions
2410 through 2430.

[0133]When the determination is made, method 2400 proceeds to the second
loop. In the second loop, method 2400 includes, at 2450, applying a
second steady state RF pulse sequence to the area of the subject. The
second steady state RF pulse sequence has no inversion pulses. It is not
a TOSSI sequence. Method 2400 also includes, at 2460, obtaining a second
echo signal in response to applying the second steady state RF pulse
sequence at 2450. Method 2400 also includes, at 2470, storing data
derived from the second echo signal in the k-space data set. The second
loop may continue until a determination is made at 2480 to acquire no
more data.

[0134]In one example, method 2400 may also include reconstructing an image
from the k-space data set. In one example, the first RF pulse sequence is
configured to produce a signal having a first strength in outer regions
of k-space. In this example, the second RF pulse sequence is configured
to produce a signal having a second strength in outer regions of k-space.
The second signal strength is greater than the first signal strength.

[0135]Acquiring data first using the TOSSI portion and then acquiring data
using a non-TOSSI portion may yield improved analytic measurements for a
reconstructed image. For example, a point spread function associated with
a k-space data set acquired using a combination of the first RF pulse
sequence and the second RF pulse sequence may have a more narrow width
than a point spread function associated with a k-space data set acquired
using only the first RF pulse sequence.

[0136]FIG. 25 illustrates a combined acquisition technique 2500. Method
2500 includes, at 2510, first controlling an MRI apparatus to apply a
first RF energy to an object for a first period of time. The first RF
energy is controlled by an initial TOSSI imaging block. Method 2500 also
includes, at 2520, controlling the MRI apparatus to subsequently apply a
second RF energy to the object for a second period of time. The second RF
energy is controlled by a set of non-inverting, non-TOSSI acquisitions.
Thus, the combined acquisition technique includes first applying a TOSSI
imaging block and then applying a non-TOSSI imaging block.

[0137]In one example, members of the set of non-inverting, non-TOSSI
acquisitions provided at 2520 are not identical. In one example, the
periods of time for which the first RF energy (TOSSI) is applied at 2510
and the second RF energy (non-TOSSI) is applied at 2520 are configurable.
In one example, the center of k-space is to be encoded during the first
period of time and the outer regions of k-space are to be encoded during
the second period of time. One skilled in the art will appreciate that
the proportions of a space denoted as a center region and as an outer
region will vary depending on the ratio between the first period of time
and the second period of time.

[0138]To the extent that the term "or" is employed in the detailed
description or claims (e.g., A or B) it is intended to mean "A or B or
both". The term "and/or" is used in the same manner, meaning "A or B or
both". When the applicants intend to indicate "only A or B but not both"
then the term "only A or B but not both" will be employed. Thus, use of
the term "or" herein is the inclusive, and not the exclusive use. See,
Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995).

[0139]To the extent that the phrase "one or more of, A, B, and C" is
employed herein, (e.g., a data store configured to store one or more of,
A, B, and C) it is intended to convey the set of possibilities A, B, C,
AB, AC, BC, and/or ABC (e.g., the data store may store only A, only B,
only C, A&B, A&C, B&C, and/or A&B&C). It is not intended to require one
of A, one of B, and one of C. When the applicants intend to indicate "at
least one of A, at least one of B, and at least one of C", then the
phrasing "at least one of A, at least one of B, and at least one of C"
will be employed.